Method for producing hollow silica particles

ABSTRACT

obtaining a hollow silica precursor from the hollow silica precursor dispersion and obtaining hollow silica particles from the hollow silica precursor.

TECHNICAL FIELD

The present invention relates to a method for producing hollow silicaparticles.

BACKGROUND ART

Hollow silica particles each comprising a shell layer formed of silicaand an empty space in the shell layer. Hollow silica particles are, dueto their variety of the particle sizes, the pore structures of the shelllayer and the surface physical properties, widely used for a catalyst, acatalyst carrier, a cosmetic pigment, a resin filler, an adsorbent, adesiccating agent, a heat insulting material, a coating material, drugdelivery system, an optical filter, etc. Further, they are useful alsoas an antireflection coating film material due to their low refractiveindex by their hollow shape.

As an example of the method for producing hollow silica particles, anoil-in-water emulsion in which an oil phase is dispersed in an aqueousphase is prepared, a silica material is deposited on oil droplets toprepare oil core/silica shell particles, and the oil droplet componentsare removed from the particles to obtain hollow silica particles. Sincethe particles are prepared from an emulsion, hollow silica particleshaving a particle size of from about several tens nm to 10 μm can beformed.

On the other hand, hollow silica particles having small particle sizeshave a thin silica shell layer, and thus the particle strengthdecreases, and the particles may be broken during use or during storage.If the shell layer has low strength, the particles may be broken whenmixed with other material such as ceramic raw material. Further, if theshell layer is porous, the hollow structure in the inside may not bemaintained when the particles are mixed with e.g. a solvent.

In Patent Document 1, an initial container bottom content containingwater, sodium chloride, precipitated calcium carbonate and sodiumsilicate is stirred at pH 9, and then an aqueous sodium silicatesolution and an aqueous sulfuric acid solution are added, followed byaging, filtration and drying to obtain dry particles, and the dryparticles are treated with concentrated hydrochloric acid to removecalcium carbonate thereby to produce hollow silica particles.

Patent Document 2 proposes hollow silica microcapsules having an averagepore diameter of from 1.6 to 10 nm. Patent Document 2 proposes a methodin which a W/O emulsion or an O/W/O emulsion containing a silicate of analkali metal in an aqueous phase is obtained, a precipitant is added tothe emulsion to form hollow silica microcapsules, followed bypredetermined washing with water, drying and baking, and further thebaked microcapsules are formed into mesoporous to produce hollow silicamicrocapsules.

Non-Patent Document 1 proposes a method in which an emulsion using apoly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) blockcopolymer and sodium silicate are combined to prepare hollow silicaparticles.

In Non-Patent Document 1, ethanol having trimethylbenzene dissolvedtherein is added to an aqueous solution having the block copolymeradded, to obtain an emulsion, to which an aqueous sodium silicatesolution is added to adjust the pH to 5.2, followed by aging, and theresulting white powder is isolated, dried and baked to prepare hollowsilica particles. By such a method, hollow silica particles having anaverage diameter of at most about 1 μm and having a BET specific surfacearea of about 426 m²/g are obtained.

PRIOR ART DOCUMENTS Non-Patent Documents

-   Non-Patent Document 1: Qianyao Sun et al, “The Formation of    Well-Defined Hollow Silica Spheres with Multilamellar Shell    Structure”, Advanced Materials, 2003, 15, No. 13, July 4.

PATENT DOCUMENTS

-   Patent Document 1: JP-A-2000-500113-   Patent Document 1: JP-A-2006-102592

DISCLOSURE OF INVENTION Technical Problem

In Patent Document 1, since the core particles are formed of calciumcarbonate, the emulsion is alkalified to deposit the silica material onthe core particles. By this method, the particle size and the shape ofthe obtainable hollow particles are restricted by the particle size andthe shape of the solid calcium carbonate particles which are the coreparticles. Further, when calcium carbonate is removed by an acid, theshell layer may have voids, and thus the shell layer cannot sufficientlybe densified, and the strength of the shell layer may decrease by theacid treatment.

In Patent Document 1, a liquid material such as a vegetable oil ismentioned as another example of the core particles, however, the silicamaterial may not sufficiently be deposited on the core particles formedof the liquid material under alkaline conditions.

Hollow silica obtained from a MO emulsion as in Patent Document 2 arenot particles having an apparent empty space in their hollow space butparticles of which the silica density is gradually decreased from theoutermost shell toward the center, since the hollow silica is formed ofdroplets of an aqueous phase containing an alkali metal silicate.

Further, hollow silica obtained from a W/O/W emulsion are such that theoil droplets at the center becomes the hollow space, and the silicamaterial contained in the aqueous phase as the intermediate layer formsa shell layer. The shell layer, which is formed of the aqueous phase inthe intermediate layer, can hardly be densified.

In Non-Patent Document 1, sodium silicate is deposited on oil dropletsof an oil-in-water emulsion under acidic conditions, followed by aging,and then the resulting white powder is isolated. By this method, theshell layer of the obtained hollow silica particles cannot sufficientlybe densified, and the hollow silica particles tend to be inferior instrength.

Under these circumstances, an object of the present invention is toprovide hollow silica particles having a dense silica shell layer.

Solution to Problem

The present invention provides the following.

[1] A method for producing hollow silica particles, which comprises:

adjusting the pH of an oil-in-water emulsion containing an aqueousphase, an oil phase and a surfactant to at most 3.0 and adding a firstsilica material to the oil-in-water emulsion,

adding a second silica material to the emulsion having the first silicamaterial added, at its pH of at least 8, in the presence of alkali metalions, to obtain a hollow silica precursor dispersion, and

obtaining a hollow silica precursor from the hollow silica precursordispersion and obtaining hollow silica particles from the hollow silicaprecursor.

[2] The method for producing hollow silica particles according to [1],wherein the first silica material and the second silica material areeach independently at least one member selected from the groupconsisting of an alkali metal silicate, activated silicic acid and asilicon alkoxide.[3] The method for producing hollow silica particles according to [2],wherein the alkali metal silicate is sodium silicate.[4] The method for producing hollow silica particles according to anyone of [1] to [3], wherein as the first silica material, an alkali metalsilicate aqueous solution is used.[5] The method for producing hollow silica particles according to anyone of [1] to [4], wherein as the second silica material, at least oneof an alkali metal silicate aqueous solution and an activated silicicacid aqueous solution is used.[6] The method for producing hollow silica particles according to anyone of [1] to [5], wherein the second silica material is added to theemulsion heated.[7] The method for producing hollow silica particles according to anyone of [1] to [6], wherein an acid is added to the oil-in-water emulsionto adjust the pH to at least 3, and then as the first silica material analkali metal silicate aqueous solution is added.[8] The method for producing hollow silica particles according to anyone of [1] to [7], wherein a base is added to the emulsion after addingthe first silica material, and then the second silica material is added.[9] The method for producing hollow silica particles according to anyone of [1] to [8], wherein the hollow silica precursor is baked toobtain the hollow silica particles.[10] The method for producing hollow silica particles according to [9],wherein the baking temperature is from 300 to 800° C.[11] The method for producing hollow silica particles according to anyone of [1] to [10], wherein the surfactant is apolyoxyethylene/polyoxypropylene copolymer.[12] The method for producing hollow silica particles according to anyone of [1] to [11], wherein the obtained hollow silica particles have anaverage primary particle size of from 10 nm to 10 μm and a BET specificsurface area of at most 300 m²/g.[13] Hollow silica particles, having an average primary particle size offrom 10 nm to 10 μm and a BET specific surface area of at most 150 m²/g,and containing an alkali metal component in an amount of at least 500mass ppm in the shell layer.

Advantageous Effects of Invention

According to the present invention, hollow silica particles having adense silica shell layer can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a transmission electron microscopic image (TEM image) of thehollow silica particles obtained in Ex. 1.

FIG. 2 is a transmission electron microscopic image (TEM image) of thehollow silica particles obtained in Ex. 1.

FIG. 3 is a scanning electron microscopic image (SEM image) of thehollow silica particles obtained in Ex. 1 after disintegrated.

FIG. 4 is a transmission electron microscopic image (TEM image) of thehollow silica particles obtained in Ex. 21.

FIG. 5 is a scanning electron microscopic image (SEM image) of thehollow silica particles obtained in Ex. 21 after disintegrated.

DESCRIPTION OF EMBODIMENTS

Now, the present invention will be described in detail. However, itshould be understood that the present invention is by no meansrestricted to the following specific description.

The method for producing hollow silica particles of the presentinvention is a method for producing hollow silica particles, whichcomprises:

adjusting the pH of an oil-in-water emulsion containing an aqueousphase, an oil phase and a surfactant to at most 3.0 and adding a firstsilica material to the oil-in-water emulsion,

adding a second silica material to the emulsion having the first silicamaterial added, at its pH of at least 8, in the presence of alkali metalions, to obtain a hollow silica precursor dispersion, and

obtaining a hollow silica precursor from the hollow silica precursordispersion and obtaining hollow silica particles from the hollow silicaprecursor.

According to the production method, hollow silica particles having adense silica shell layer can be obtained. Since the shell layer isdense, the particle strength is increased, and their shape can bemaintained even when mixed with other material. Further, since the shelllayer is dense, the solvent and foreign substances from outside are lesslikely to invade the hollow portion in the inside.

Further, the silica material is not limited, and for example, even analkali metal silicate may be used to provide hollow silica particleshaving a dense silica shell layer.

In this method, an oil-in-water emulsion containing an aqueous phase, anoil phase and a surfactant is used. This oil-in-water emulsion is anemulsion having an oil phase dispersed in water, and when a silicamaterial is added to the emulsion, the silica material is deposited onthe oil droplets, whereby oil core/silica shell particles can be formed.Hereinafter, the oil-in-water emulsion may sometimes be referred tosimply as an emulsion.

Further, a dispersion having oil core/silica shell particles dispersed,formed by adding the first silica material and not having the secondsilica material added yet, and a dispersion having oil core/silica shellparticles dispersed, having the second silica material added, may alsobe sometimes referred to as an emulsion. The latter dispersion havingoil core/silica shell particles dispersed, having the second silicamaterial added may be the same as the hollow silica precursordispersion.

The aqueous phase of the emulsion contains mainly water as a solvent.The aqueous phase may further contain a water-soluble organic liquid oran additive such as a water-soluble resin. The proportion of the waterin the aqueous phase is preferably from 50 to 100 mass %, morepreferably from 90 to 100 mass %.

The oil phase of the emulsion preferably contains a water-insolubleorganic liquid which is incompatible with the aqueous phase component.This organic liquid forms droplets in the emulsion thereby to form anoil core portion of the hollow silica precursor.

The organic liquid may, for example, be an aliphatic hydrocarbon such asn-hexane, isohexane, n-heptane, isoheptane, n-octane, isooctane,n-nonane, isononane, n-pentane, isopentane, n-decane, isodecane,n-dodecane, isododecane or pentadecane, a paraffin base oil which is amixture thereof, an alicyclic hydrocarbon such as cyclopentane,cyclohexane or cyclohexene, a naphthene base oil which is a mixturethereof, an aromatic hydrocarbon such as benzene, toluene, xylene,ethylbenzene, propylbenzene, cumene, mesitylene, tetralin or styrene, anether such as propyl ether or isopropyl ether, an ester such as ethylacetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutylacetate, n-amyl acetate, isoamyl acetate, butyl lactate, methylpropionate, ethyl propionate, butyl propionate, methyl butyrate, ethylbutyrate or butyl butyrate, a vegetable oil such as palm oil, soybeanoil or rapeseed oil, or a fluorinated solvent such as ahydrofluorocarbon, a perfluorocarbon or a perfluoropolyether. Further, apolyoxyalkylene glycol to be a hydrophobic liquid at the shell formingreaction temperature may also be used. For example, polypropylene glycol(molecular weight: at least 1,000), or apolyoxyethylene/polyoxypropylene block copolymer having a proportion ofoxyethylene units of less than 20 mass % and a cloudy point (1 mass %aqueous solution) of at most 40° C., preferably at most 20° C. may bementioned. Among them, apolyoxypropylene/polyoxyethylene/polyoxypropylene type block copolymeris preferably used.

They may be used alone or in combination of two or more so long as asingle oil phase is formed.

The organic liquid is preferably a C₈₋₁₆, particularly C₉₋₁₂hydrocarbon. The organic liquid is selected comprehensively consideringthe workability, the safety against fire, separation property of thehollow silica precursor and the organic liquid, the shape of the hollowsilica particles, solubility of the organic liquid in water, etc. TheC₈₋₁₆ hydrocarbon may be any of linear, branched and cyclic hydrocarbonsso long as it is chemically stable, or may be a mixture of hydrocarbonsdiffering in the number of carbon atoms. The hydrocarbon is preferably asaturated hydrocarbon, more preferably a linear saturated hydrocarbon.

The organic liquid is preferably one having a flash point of from 20 to90° C., more preferably from 30 to 80° C. If an organic liquid having aflash point of less than 20° C. is used, countermeasures in the workenvironment are required for fire control due to too low flash point.Further, if an organic liquid having a flash point higher than 90° C. isused, due to small volatility, the amount of the organic liquiddeposited on the obtained hollow silica particles may be large.

The emulsion contains a surfactant so as to improve emulsificationstability. The surfactant is preferably water-soluble orwater-dispersible, and is used preferably as added to the aqueous phase.The surfactant is preferably a nonionic surfactant.

As the nonionic surfactant, for example, the following surfactants maybe mentioned.

Polyoxyethylene/polyoxypropylene copolymer type surfactants

Polyoxyethylene sorbitan fatty acid ester type surfactants:polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitanmonopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylenesorbitan tristearate, polyoxyethylene sorbitan monooleate

Polyoxyethylene fatty acid alcohol ether type surfactants:polyoxyethylene lauryl ether, polyoxyethylene cetyl ether,polyoxyethylene stearyl ether, polyoxyethylene oleyl ether,polyoxyethylene octyl phenol ether, polyoxyethylene nonyl phenol ether

Polyoxyethylene fatty acid ester type surfactants: polyoxyethyleneglycol monolaurate, polyoxyethylene glycol monostearate, polyoxyethyleneglycol monooleate

Glycerin fatty acid ester type surfactants: monoglyceryl stearate,monoglyceryl oleate Further, a polyoxyethylene sorbitol fatty acid estertype surfactant, a sugar ester type surfactant, a polyglycerin fattyacid ester type surfactant, a polyoxyethylene hydrogenated caster oiltype surfactant or the like may also be used.

They may be used alone or in combination of two or more.

Among the above-mentioned nonionic surfactants, thepolyoxyethylene/polyoxypropylene copolymer type surfactant is preferablyused. The polyoxyethylene/polyoxypropylene copolymer is a blockcopolymer having a polyoxyethylene block (EO) and a polyoxypropyleneblock (PO) bonded. The block copolymer may, for example, be an EO-PO-EOblock copolymer or an EO-PO block copolymer, and is preferably anEO-PO-EO block copolymer. The proportion of oxyethylene units in theEO-PO-EO block copolymer is preferably at least 20 mass %, morepreferably at least 30 mass %.

The weight average molecular weight of thepolyoxyethylene/polyoxypropylene copolymer is preferably from 3,000 to27,000, more preferably from 6,000 to 19,000.

To the entire polyoxyethylene/polyoxypropylene copolymer, the totalamount of the polyoxyethylene block is preferably from 40 to 90 mass %,and the total amount of the polyoxypropylene block is preferably from 10to 60 mass %.

The amount of the surfactant used varies depending upon the conditionssuch as the type of the surfactant, HLB (hydrophile-lipophile balance)which is an index indicating the degree of hydrophilicity orhydrophobicity of the surfactant, and the particle size of the silicaparticles desired, and as the content in the aqueous phase, it ispreferably from 500 to 20,000 mass ppm, more preferably from 1,000 to10,000 mass ppm. When it is at least 500 mass ppm, the emulsion can bemore stabilized. Further, when it is at most 20,000 mass ppm, the amountof the surfactant remaining in the hollow silica particles as a productcan be reduced.

The aqueous phase and the oil phase may be blended in a mass ratio offrom 200:1 to 5:1, preferably from 100:1 to 9:1.

The method of preparing the oil-in-water emulsion is not limited to thefollowing description. The aqueous phase and the oil phase arepreliminarily adjusted, and the oil phase is added to the aqueous phase,followed by sufficient mixing or stirring, whereby the oil-in-wateremulsion can be prepared. Further, ultrasonic emulsification ofphysically applying a strong shearing force, emulsification by stirring,high pressure emulsification or the like may be employed. Further,membrane emulsification method of forcing the oil phase through amembrane having fine pores to form fine droplets of the oil phase, whichare dispersed in the aqueous phase, phase inversion emulsificationmethod of dissolving the surfactant in the oil phase and then adding theaqueous phase and conducting emulsification, or a phase inversiontemperature emulsification method of utilizing a phenomenon that thesurfactant changes from water-soluble to oil-soluble at a temperaturearound the cloudy point may, for example, be mentioned. Theemulsification method may be properly selected depending upon thedesired particle size, particle size distribution, etc.

In order to decrease the particle size of the obtainable hollow silicaparticles and to narrow the particle size distribution, it is preferredthat the oil phase is sufficiently dispersed in the aqueous phase andthe mixed liquid is emulsified. For example, the mixed liquid may beemulsified by means of a high pressure homogenizer under a pressure ofat least 100 bar, preferably at least 400 bar.

In the present method, the first silica material is added to theoil-in-water emulsion.

The first silica material may, for example, be an aqueous solutionhaving water-soluble silica dissolved, an aqueous dispersion havingsolid silica dispersed, a mixture thereof, or at least one memberselected from the group consisting of an alkali metal silicate,activated silicic acid and a silicon alkoxide, or an aqueous solution oraqueous dispersion thereof. Among them, at least one member selectedfrom the group consisting of an alkali metal silicate, activated silicicacid and a silicon alkoxide, or an aqueous solution or aqueousdispersion thereof, is preferred in view of availability.

The solid silica may, for example, be silica sol obtained by hydrolyzingan organic silicon compound, or commercially available silica sol.

The alkali metal of the alkali metal silicate may, for example, belithium, sodium, potassium or rubidium, and among them, in view ofavailability and the cost, sodium is preferred. That is, alkali metalsilicate is preferably sodium silicate. Sodium silicate has acomposition represented by Na₂O.nSiO₂.mH₂O. The proportion of sodium tosilicate is, by the molar ratio n of Na₂O/SiO₂, preferably from 1.0 to4.0, further preferably from 2.0 to 3.5.

The activated silicic acid is one obtained by subjecting an alkali metalsilicate to cation exchange to replace the alkali metal with hydrogen,and the aqueous solution of the activated silicic acid is weakly acidic.For the cation exchange, a hydrogen form cation exchange resin may beused.

The alkali metal silicate or the activated silicic acid is preferablyadded to the emulsion as dissolved or dispersed in water. Theconcentration of the alkali metal silicate or activated silicic acidaqueous solution is, as the SiO₂ concentration, preferably from 3 to 30mass %, more preferably from 5 to 25 mass %.

The silicon alkoxide may, for example, be a tetraalkoxysilane such astetramethoxysilane, tetraethoxysilane or tetrapropoxysilane.

Further, by mixing other metal oxide or the like with the silicamaterial, composite particles may be obtained. Such other metal oxidemay, for example, be titanium dioxide, zinc oxide, cerium oxide, copperoxide, iron oxide or lead oxide.

As the first silica material, the silica material may be used alone oras a mixture of two or more. Among them, as the first silica material,an alkali metal silicate aqueous solution, particularly a sodiumsilicate aqueous solution is preferably used.

In the present method, addition of the first silica material to theoil-in-water emulsion is carried out at a pH of the oil-in-wateremulsion of at most 3.0. It is preferred to achieve a pH of at most 3.0by adding an acid to the oil-in-water emulsion containing the aqueousphase, the oil phase and the surfactant.

As an example of addition of the first silica material, an acid is addedto the emulsion and then the sodium silicate aqueous solution is added.

Since a neutral emulsion is once acidified, and then the sodium silicateaqueous solution as an alkali component is added, the entire emulsioncan be kept acidic at the time of adding the first silica material.

At the time of adding the first silica material, it is preferred thatthe pH of the emulsion after adding the acid to the emulsion is adjustedto at most 2, and then the pH is adjusted to at most 3.0 after addingthe sodium silicate aqueous solution.

The pH at the time of adding the first silica material to the emulsionis preferably at most 3.0, more preferably at most 2.4, whereby information of a first layer coating film by the silica material on theoil droplets in the emulsion via the surfactant, the thickness of thecoating film can be made more uniform, and the silica shell layer of theobtainable hollow silica can be made more dense.

The pH at the time of adding the first silica material to the emulsionmay be at least 1.

The acid may, for example, be hydrochloric acid, nitric acid, sulfuricacid, acetic acid, perchloric acid, hydrobromic acid, trichloroaceticacid, dichloroacetic acid, methanesulfonic acid or benzenesulfonic acid.

At the time of adding the first silica material, as the amount of thefirst silica material added, the amount of SiO₂ in the first silicamaterial is preferably from 1 to 50 parts by mass, more preferably from3 to 30 parts by mass per 100 parts by mass of the oil phase containedin the emulsion.

At the time of adding the first silica material, it is preferred tomaintain the pH of the emulsion after adding the first silica materialto be at most 3.0 for at least 1 minute, more preferably at least 5minutes, further preferably at least 10 minutes.

Then, it is preferred to maintain the pH of the emulsion having thefirst silica material added to be at least 5, whereby the first silicamaterial can be fixed to the surface of the oil droplets.

For example, the pH of the emulsion may be kept to be at least 5 byadding a base to the emulsion having the first silica material added.

The base may, for example, be an alkali metal hydroxide such as sodiumhydroxide or potassium hydroxide, an alkaline earth metal hydroxide suchas magnesium hydroxide or calcium hydroxide, ammonia or an amine.

Otherwise, a method of replacing an anion such as a halogen ion with ahydroxide ion by anion exchange may be employed.

At the time of adding the base, it is preferred to gradually add thebase while the emulsion having the silica material added is stirred soas to gradually increase the pH of the emulsion. If stirring is notintense, or a large amount of the base is added all at once, the pH ofthe emulsion tends to be non-uniform, and the thickness of the firstlayer coating film may be non-uniform.

It is preferred to maintain the emulsion at a pH of at least 5preferably with stirring. The retention time may be at least 10 minutes,and is preferably at least one hour, and may be at least 4 hours.

While the emulsion is maintained, the pH of the emulsion is preferablyat most 7.

Then, the second silica material is added at a pH of the emulsion of atleast 8 in the presence of alkali metal ions, whereby the hollow silicaprecursor dispersion is obtained. The hollow silica precursor is in theform of oil core/silica shell particles.

As the second silica material, a material similar to the first silicamaterial may be used alone or as a mixture of two or more. Particularly,as the second silica material, at least one of the sodium silicateaqueous solution and the activated silicic acid aqueous solution may bepreferably used.

To add the second silica material at a pH of the emulsion of at least 8,a method of adding an alkali metal hydroxide simultaneously withaddition of the second silica material may be employed. Otherwise, amethod of using sodium silicate as the alkali metal silicate for thesecond silica material may also be employed. In such a case, a sodiumsilicate component as an alkali component is added to the weakly acidicemulsion having the pH adjusted to be at least 5 after adding the firstsilica material, and accordingly the emulsion can be maintained to bealkaline with a pH of at least 8 while the second silica material isadded. Further, the alkali metal ions will be present in the emulsion.

The pH of the emulsion when the second silica material is added to theemulsion is preferably at least 8, and may be at least 9, whereby on thefirst layer coating film by the first silica material, a more densesecond layer coating film can be formed.

At the time of adding the first silica material, the method of onceacidifying the emulsion and then adjusting the pH to be at least 5 isemployed, so that the first silica material is more uniformly depositedon the oil droplets. The first silica layer obtained by such a method isporous and has insufficient denseness and thereby has low strength. Byalkalifying the emulsion at the time of adding the second silicamaterial, a high density second silica layer can be formed on theobtained first silica layer. From the silica layer formed in two stages,a dense silica shell layer can be formed.

The pH of the emulsion when the second silica material is added to theemulsion is not particularly limited, and may be at most 13, and may beat most 11. In a case where the pH is too high e.g. in a case where thesodium silicate aqueous solution is used as the second silica material,an acid may be added to adjust the pH. The acid to be used may be thesame acid as that used when the first silica material is added.

Addition of the second silica material is carried out in the presence ofalkali metal ions. The alkali metal ions may be derived from the firstsilica material, may be derived from the second silica material, or maybe derived from the base added to adjust the pH, or may be incorporatede.g. by addition of an additive to the emulsion. For example, an alkalimetal silicate is used as at least one of the first silica material andthe second silica material. Otherwise, as the additive to the emulsion,a halide, sulfate, nitrate, aliphatic acid salt or the like of an alkalimetal is used.

As the second silica material, for example, at least one of the sodiumsilicate aqueous solution and the activated silicic acid aqueoussolution may be added to the emulsion after adding the first silicamaterial, or both may be added. In a case where both are added, thesodium silicate aqueous solution and the activated silicic acid aqueoussolution may be added all at once, or may be added in order.

Addition of the second silica material may be carried out, for example,by conducting a step of adding the sodium silicate aqueous solution anda step of adding the activated silicic acid aqueous solution once orrepeatedly two or more times, so as to promote deposition of the silicamaterial on the first silica layer while the pH is adjusted.

The second silica material is preferably added to the emulsion heated soas to promote deposition of the silica material on the first silicalayer. The heating temperature is preferably from 30 to 100° C., morepreferably from 50 to 80° C. In a case where the heated emulsion isused, after adding the second silica material, the formed emulsion ispreferably gradually cooled to room temperature (23° C.).

At the time of adding the second silica material, the amount of thesecond silica material added is preferably adjusted so that the amountof SiO₂ in the second silica material is from 20 to 500 parts by mass,more preferably from 40 to 300 parts by mass per 100 parts by mass ofthe oil phase.

At the time of adding the second silica material, it is preferred tomaintain the emulsion after adding the second silica material to a pH ofat least 8 for at least 10 minutes.

By the addition of the first silica material and the addition of thesecond silica material, the total amount of the first silica materialand the second silica material added is preferably adjusted so that thetotal amount of SiO₂ in the first silica material and SiO₂ in the secondsilica material is from 30 to 500 parts by mass, more preferably from 50to 300 parts by mass per 100 parts by mass of the oil phase.

The silica shell layer of the present invention may be constitutedmainly by silica, and as the case requires, other metal component suchas Ti or Zr may be incorporated e.g. for the refractive indexadjustment. The method of incorporating other metal component is notparticularly limited, and for example, in the step of adding the silicamaterial, a metal sol or a metal salt aqueous solution is addedsimultaneously.

Now, the step of obtaining a hollow silica precursor from the hollowsilica precursor dispersion and then obtaining hollow silica particlesfrom the hollow silica precursor will be described.

As the method of obtaining the hollow silica precursor from the hollowsilica precursor dispersion, for example, a method of subjecting thedispersion to filtration, a method of removing the aqueous phase byheating, or a method of separating the precursor by sedimentation orcentrifugal separation may be mentioned.

As an example, a method of subjecting the dispersion to filtrationthrough a filter of from about 0.1 μm to about 5 μm, and drying thecollected hollow silica precursor may be mentioned.

Further, as the case requires, the obtained hollow silica precursor maybe washed with water, an acid, an alkali, an organic solvent or thelike.

As the method of removing the oil core from the hollow silica precursorto obtain hollow silica particles, for example, a method of baking thehollow silica precursor so that the oil is burnt and decomposed, amethod of volatilizing the oil by drying, a method of adding anappropriate additive to decompose the oil, or a method of extracting theoil e.g. with an organic solvent may be mentioned.

As an example, it is preferred to heat the hollow silica precursor at aheating temperature of from 300° C. to 800° C., particularly at from400° C. to 600° C. for a heating time of from 1 to 8 hours, particularlyfor from 3 to 6 hours. In such a case, the temperature-raising rate ispreferably from 1 to 20° C./min, particularly from 2 to 10° C./min.

The obtained hollow silica particles may be aggregated by the drying orbaking step, and thus may be disintegrated into a handleable aggregationsize. The disintegration method may, for example, be a method using amortar, a method using a dry or wet ball mill, a method of using a sieveshaker, or a method of using a disintegrator such as a pin mill, acutter mill, a hammer mill, a knife mill or a roller mill may, forexample, be mentioned.

By the obtained hollow silica particles having an empty space in theshell layer can be confirmed by transmission electron microscopic image(TEM) observation. Spherical particles having an empty space in theinside confirmed by TEM observation are defined as “primary particles”.Since the primary particles are partially bonded in the baking or dryingstep, the obtained hollow silica is in the form of aggregates ofsecondary particles having the primary particles aggregated in manycases.

The size of the primary particles is obtained by directly observing theparticle sizes by TEM observation. Specifically, a portion at which thedistribution of the primary particle size in an observation area isconsidered to be at the same level as distribution of the primaryparticle size of the entire obtained hollow silica particles, withoutparticles having extremely large or small particle sizes, is observed asenlarged to measure the sizes of the respective primary particles, andthe distribution of the sizes of the primary particles obtained from thesizes of the respective primary particles is estimated to be thedistribution of the sizes of the entire primary particles.

The average of the sizes of the primary particles is preferably from 10nm to 10 μm, more preferably from 50 nm to 2 μm, further preferably from100 nm to 1 μm.

The hollow silica particles preferably have a BET specific surface areaof at most 300 m²/g, more preferably at most 200 m²/g, furtherpreferably at most 150 m²/g.

The BET specific surface area is measured by multi-point method usingliquid nitrogen, by a specific surface area measuring apparatus “TriStar113020” manufactured by Shimadzu Corporation, with respect to the hollowsilica particles dried to 50 mTorr at 230° C. as a pre-treatment.

The shell thickness of the hollow silica particles relative to the sizeof the primary particles is preferably from 0.01 to 0.3, more preferablyfrom 0.02 to 0.2, further preferably from 0.03 to 0.1.

If the shell thickness is smaller than 0.01 relative to the size of theprimary particles, the strength of the hollow silica particles maydecrease. If the ratio is larger than 0.3, the empty space in the insidetends to be small, and properties of the hollow silica particles beinghollow cannot be obtained.

The shell thickness is obtained by measuring the shell thicknesses ofthe respective particles by TEM observation in the same manner as theprimary particle size.

The hollow silica particles, which are formed by using oil droplets ofthe oil-in-water emulsion as core particles, are spherical, preferablytrue spherical.

In the present invention, the alkali metal component is contained in theshall layer of the obtained hollow silica particles. Substantially noalkali metal component is measured when the silicon alkoxide is used asthe silica material.

For example, in a case where the sodium silicate aqueous solution isused as the silica material, the mass concentration of the Na componentin the shell of the obtained hollow silica particles is at least 500mass ppm, or in a higher case, at least 1,000 mass ppm. On the otherhand, in the shell of conventional hollow silica particles prepared byusing tetraethyl orthosilicate as the silica material, the massconcentration of the Na component is at most 100 mass ppm.

The Na component may be measured by ICP spectrometry with respect to asample obtained by adding perchloric acid and hydrofluoric acid to theobtained hollow silica and strongly heating the silica to remove siliconas the main component.

Further, in a case where the alkali metal silicate is used as the silicamaterial, the carbon (C) component derived from the material in theshell layer of the obtained hollow silica particles tends to be small ascompared with a case where the silicon alkoxide is used as the silicamaterial.

The hollow silica particles produced in accordance with the method ofthe present invention are characterized by the denseness of the shelllayer, and have, for example, a property such that they sediment inwater but float in an oil component.

Part of the hollow silica particles produced in accordance with themethod of the present invention may be single-hole hollow silica (hollowsilica having one hole). This hole is observed by an electron microscope(SEM) and preferably has a hole size of form 5 nm to 3 μm and at most ⅓of the particle size of the hollow silica particles, preferably from 10nm to 1 μm and at most ⅕ of the particle size of the hollow silicaparticles.

Such single-hole hollow silica may be utilized as a modified-releasedosage form of perfume or chemicals. That is, a predetermined chemicalor the like (preferably liquid or solution) is incorporated into theparticles under reduced pressure. The chemical or the like is releasedin a controlled release manner from the single hole under normalpressure.

The single-hole hollow silica particles may be selected from theparticle group of the present invention by the following method forexample. First, the hollow silica particles are charged into a liquidmedium under atmospheric pressure, and particles floating up arerecovered. By using a fluorinated solvent as the liquid medium, aparticle group having an apparent specific gravity lower than the liquidmedium is easily recovered. Then, the recovered particle group ischarged into a liquid medium under reduced pressure. Single-hole hollowsilica particles sediment in the liquid medium since the liquid mediuminfiltrates into the hollow space. The particles which had sedimented inthe liquid medium are recovered and dried, whereby a single-hole hollowsilica particle group is recovered.

The hollow silica particles of the present invention have an averageprimary particle size of from 10 nm to 10 μm and a BET specific surfacearea of at most 300 m²/g, and contains the alkali metal component in theshell layer.

The mass concentration of the alkali metal component in the shell of thehollow silica particles obtained by the present invention is preferablyat least 500 mass ppm, more preferably at least 1,000 mass ppm. If thealkali metal component is small, the shell tends to be porous, and adense shell having high strength cannot be obtained.

The hollow silica particles of the present invention may be produced bythe above production method, but are not limited to one produced by theproduction method.

The details of the average primary particle size, the BET specificsurface area, etc. of the hollow silica particles are as mentionedabove.

EXAMPLES

Now, the present invention will be described in further detail withreference to Examples, but the present invention is not limited thereto.In the following description, as components in common, the samecomponent is used. Further, unless otherwise specified, “%” means “mass%”. Ex. 1 to 17 are Examples of the present invention, and Ex. 21 is aComparative Example.

Ex. 1

(Preparation of Emulsion)

3.6 g of an EO-PO-EO block copolymer (Kolliphor P188 manufactured byBASF) was added to 1,778 g of pure water, followed by stirring untildissolution.

To the obtained aqueous solution, 18 g of n-dodecane (reagent chemical,manufactured by Wako Pure Chemical Industries, Ltd.) was added, followedby stirring by a homogenizer manufactured by IKA until the entire liquidbecame uniform to prepare a crude emulsion.

The crude emulsion was emulsified three times under a pressure of 400bar by using a high pressure emulsifier (LAB1000 manufactured by SMTCO., LTD.) to prepare a fine emulsion.

(First Stage Shell Formation)

To 1,600 g of the obtained fine emulsion, 3.4 g of 35% hydrochloric acid(reagent chemical, manufactured by Wako Pure Chemical Industries, Ltd.)was added to adjust the pH to 1.7.

Then, 34.8 g of a diluted sodium silicate aqueous solution (SiO₂concentration: 6.9 mass %, Na₂O concentration: 2.3 mass %) was added,followed by thorough stirring, and the liquid was maintained at a pH of2.4 for 15 minutes.

46.1 g of a 0.1 M sodium hydroxide aqueous solution was slowly dropwiseadded to the liquid with thorough stirring, and stirring was continuedfor 4 hours, whereby an oil core/silica shell particle dispersion havinga pH of 5.2 was obtained.

(Preparation of Activated Silicic Acid)

To 270 g of a well washed hydrogen form cation exchange resin (SK1 BHmanufactured by Mitsubishi Plastics, Inc.), 380 g of pure water wasadded, and the temperature was decreased to 5° C.

To the resin dispersion kept at a temperature of 5° C., 360 g of adiluted sodium silicate aqueous solution (SiO₂ concentration: 10.3 mass%, Na₂O concentration: 3.5 mass %) was dropwise added little by littlewith thorough stirring.

After completion of dropwise addition, the resin was removed byfiltration to obtain a 5 mass % activated silicic acid aqueous solution.

(Second Stage Shell Formation)

1,300 g of the oil core/silica shell particle dispersion obtained in thefirst stage shell formation was heated to 70° C., and 35.2 g of adiluted sodium silicate aqueous solution (SiO₂ concentration: 3.5 mass%, Na₂O concentration: 1.2 mass %) was added with stirring to adjust thepH to 9.6.

200 g of the above prepared 5 mass % activated silicic acid aqueoussolution was slowly dropwise added to the suspension kept at 70° C. withstirring to adjust the pH to 9.2.

Then, 1.4 g of a diluted sodium silicate aqueous solution (SiO₂concentration: 3.5 mass %, Na₂ O concentration: 1.2 mass %) was added toadjust the pH to 9.5.

Then, 200 g of the 5 mass % activated silicic acid aqueous solution wasslowly dropwise added to adjust the pH to 9.2.

5.2 g of a diluted sodium silicate aqueous solution (SiO₂ concentration:3.5 mass %, Na₂ O concentration: 1.2 mass %) was added again to adjustthe pH to 9.4.

195 g of the 5 mass % activated silicic acid aqueous solution was slowlydropwise added again to adjust the pH to 8.9. The liquid was slowlycooled to room temperature to obtain a hollow silica precursordispersion.

(Filtration, Drying, Baking)

1,200 g of the hollow silica precursor dispersion was subjected topressure filtration (pressure: 0.28 MPa) through a 0.45 μm hydrophilicPTFE membrane filter, followed by drying at 80° C. for 8 hours to obtaina hollow silica precursor.

The obtained precursor was baked at 550° C. for 4 hours(temperature-raising rate: 10° C./min) to obtain 6 g of hollow silicaparticles.

(Evaluation)

The BET specific surface area of the hollow silica obtained in Ex. 1 bynitrogen adsorption method was 118 m²/g.

Transmission electron microscopic images (TEM images) of the hollowsilica particles obtained in Ex. 1 are shown in FIGS. 1 and 2.

116 primary particles having clear contours were selected from the TEMimage shown in FIG. 1, and their diameters were measured and calculated,whereupon the average was 278 nm. Further, the thickness of the shell inthe TEM image shown in FIG. 2 measured was 15 nm.

A scanning electron microscopic image (SEM image) of the hollow silicaparticles obtained in Ex. 1 disintegrated by an agate mortar is shown inFIG. 3. It is found from FIG. 3 that the hollow silica particles in Ex.1 maintained the shape of the hollow particles even after disintegratedin the mortar.

The mass concentration of Na in the shell of the hollow silica obtainedin Ex. 1 was 3,800 mass ppm.

Ex. 21

(Preparation of Emulsion)

18 g of an EO-PO-EO block copolymer (Kolliphor P188 manufactured byBASF) was added to 1,765 g of pure water, followed by stirring untildissolution.

To the obtained aqueous solution, 18 g of n-dodecane was added, followedby stirring by a homogenizer manufactured by IKA until the entire liquidbecomes uniform to prepare a crude emulsion.

The crude emulsion was emulsified three times under a pressure of 400bar by using a high pressure emulsifier (LAB1000 manufactured by SMTCO., LTD.) to prepare a fine emulsion.

(Formation of Hollow Silica Precursor)

9.2 g of 35% hydrochloric acid was added to 1,700 g of the obtained fineemulsion to adjust the pH to 1.5.

Then, 123.3 g of a diluted sodium silicate aqueous solution (SiO₂concentration: 6.9 mass %, Na₂O concentration: 2.3 mass %) was added,followed by thorough stirring, and the liquid was maintained at a pH of2.2 for 15 minutes.

127.1 g of a 0.1 M sodium hydroxide aqueous solution was slowly dropwiseadded to the liquid with thorough stirring, and stirring was continuedfor 4 hours, whereupon a hollow silica precursor dispersion having a pHof 5.2 was obtained.

(Filtration, Drying, Baking)

600 g of the hollow silica precursor dispersion was subjected topressure filtration through a 0.45 μm hydrophilic PTFE membrane filter,followed by drying at 80° C. for 8 hours to obtain a hollow silicaprecursor.

The obtained precursor was baked at 550° C. for 4 hours(temperature-raising rate: 10° C./min) to obtain 3 g of hollow silicaparticles.

(Evaluation)

The BET specific surface area of the hollow silica obtained in Ex. 21 bynitrogen adsorption method was 554 m²/g.

A TEM image of the hollow silica particles obtained in Ex. 21 is shownin FIG. 4.

In the TEM image shown in FIG. 4, the shell layer is vague as comparedwith that in the TEM image shown in FIG. 2, thus indicating a lowdenseness of the shell as compared with Ex. 1.

A SEM image of the hollow silica particles obtained in Ex. 21disintegrated by an agate mortar is shown in FIG. 5. It is found fromFIG. 5 that most of the hollow silica particles in Ex. 21 collapsedsince the silica shell strength was insufficient, and fragments of thesilica shell were present in a large amount.

Ex. 2

(Preparation of Emulsion)

2.4 g of an EO-PO-EO block copolymer (Kolliphor P188 manufactured byBASF) was added to 480 g of pure water, followed by stirring untildissolution. To the obtained aqueous solution, 16 g of n-dodecane wasadded, followed by stirring by a homogenizer manufactured by IKA untilthe entire liquid became uniform to prepare a crude emulsion.

The crude emulsion was emulsified three times under a pressure of 400bar by using a high pressure emulsifier (LAB1000 manufactured by SMTCO., LTD.) to prepare a fine emulsion.

(First Stage Shell Formation)

To 442 g of the obtained fine emulsion, 9 g of 2M hydrochloric acid wasadded to adjust the pH to 1.5.

Then, 12.3 g of a diluted sodium silicate aqueous solution (SiO₂concentration: 10.4 mass %, Na₂O concentration: 3.6 mass %) was added,followed by thorough stirring, and the liquid was maintained at a pH of2.1 for 15 minutes.

4 g of a 0.1 M sodium hydroxide aqueous solution was slowly dropwiseadded to the liquid with thorough stirring, and stirring was continuedfor 1 hour, whereby an oil core/silica shell particle dispersion havinga pH of 5.8 was obtained.

(Second Stage Shell Formation)

400 g of the oil core/silica shell particle dispersion obtained in Firststage shell formation was heated to 30° C., and 3 g of a diluted sodiumsilicate aqueous solution (SiO₂ concentration: 10.4 mass %, Na₂Oconcentration: 3.6 mass %) was slowly added with stirring to adjust thepH to 9.

Then, 127 g of a diluted sodium silicate aqueous solution (SiO₂concentration: 10.4 mass %, Na₂ O concentration: 3.6 mass %) wasgradually added together with 0.5M hydrochloric acid to adjust the pH to9.

The suspension was maintained at 30° C. for 2 days and slowly cooled toroom temperature to obtain a hollow silica precursor dispersion.

(Filtration, Drying, Baking)

770 g of the hollow silica precursor dispersion was subjected topressure filtration (pressure: 0.28 MPa) through a 0.45 μm hydrophilicPTFE membrane filter.

The cake collected by filtration was dried in a nitrogen atmosphere at60° C. for 1 hour and at 400° C. for 4 hours (temperature-raising rate:5° C./min) to obtain a hollow silica precursor.

The obtained precursor was baked at 550° C. for 4 hours(temperature-raising rate: 5° C./min) to obtain 13.8 g of hollow silicaparticles.

The specific surface area of the hollow silica particles obtained in Ex.2 is shown in Table 1.

Ex. 3 to 5

Silica particles were prepared in the same manner as in Ex. 2 exceptthat the reaction temperature and the retention temperature in Secondstage shell formation were changed.

The reaction temperature and the retention temperature in Second stageshell formation and the specific surface area of the obtained hollowsilica particles in Ex. 3 to 5 are shown in Table 1.

The higher the reaction temperature and the retention temperature inSecond stage shell formation, the smaller the specific surface area,thus indicating that hollow silica particles having a dense shell can beprepared.

TABLE 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Reaction temperature and retention 30 5060 70 temperature (° C.) in Second stage shell formation Specificsurface area (m²/g) 298 272 160 60

Ex. 6 to 13

Silica particles were prepared in the same manner as in Ex. 5 exceptthat the EO-PO-EO block copolymer was changed. EO-PO-EO block copolymersused in the respective Ex. are shown in Table 2.

Hollow silica particles could be obtained in all of Ex. 6 to 13,however, in Ex. 6, 7 and 11, hollow silica particles having one holewere observed in a large amount.

TABLE 2 EO-PO-EO block copolymer used Molec- EO Specific Manu- ularpropor- surface factured Product Product weight tion area by name No.(g/mol) (wt %) (m²/g) Ex. 6 BASF Pluronic PE 6500 50 70 10500 Ex. 7 PE8125 40 85 10400 Ex. 8 ADEKA Pluronic F-108 16250 80 38 Ex. 9 F-88 1125080 76 Ex. 10 F-68 8750 80 37 Ex. 11 P-85 11250 50 102 Ex. 12 NOF PLONON#208 8600 76 95 Ex. 13 CORPORA- #188 8310 80 48 TION

Ex. 14

Silica particles were prepared in the same manner as in Ex. 5 exceptthat the amount of the EO-PO-EO block copolymer (Kolliphor P188manufactured by BASF) added was 10 times.

The specific surface area of the hollow silica particles obtained in Ex.14 was 140 m²/g.

Ex. 15

(Preparation of Emulsion)

2.4 g of an EO-PO-EO block copolymer (F-68 manufactured by ADEKACorporation) was added to 480 g of pure water, followed by stirringuntil dissolution. 16 g of n-dodecane was added to the aqueous solution,followed by stirring by a homogenizer manufactured by IKA until theentire liquid becomes uniform, to prepare a crude emulsion.

Ultrasonic irradiation was conducted twice to the crude emulsion by anultrasonic dispersing machine (GSCVP-600 manufactured by SonicTechnology Co., Ltd.) at an intensity of V-Level 3.4 for one minute toprepare a fine emulsion.

First stage shell formation and subsequent steps were conducted in thesame manner as in Ex. 5 to prepare silica particles, except that afterFirst stage shell formation, the liquid having the silica material addedwas not maintained at pH 5, and a 1M sodium hydroxide aqueous solutionwas added to pH 9.

The specific surface area of the hollow silica particles obtained in Ex.15 was 170 m²/g.

Ex. 16

(Preparation of emulsion)

4.9 g of an EO-PO-EO block copolymer (Pluronic PE10400 manufactured byBASF) was added to 462 g of pure water, followed by stirring untildissolution.

33 g of ASAHIKLIN AC-6000 (manufactured by AGC Inc.,1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluorooctane) as a fluorinated solventwas added to the aqueous solution, followed by stirring by a homogenizermanufactured by IKA until the entire liquid becomes uniform, to preparea crude emulsion.

Ultrasonic irradiation was conducted twice to the crude emulsion by anultrasonic dispersing machine (GSCVP-600 manufactured by SonicTechnology Co., Ltd.) at an intensity of V-Level 3.4 for one minute toprepare a fine emulsion.

(First Stage Shell Formation), (Second Stage Shell Formation),(Filtration)

First stage shell formation and Second stage shell formation, andFiltration were conducted in the same manner as in Ex. 5 to preparesilica particles.

(Drying)

The cake collected by filtration was dried at 60° C. for 2 days toremove AC-6000 thereby to obtain 8 g of hollow silica particles.

The specific surface area of the hollow silica particles obtained in Ex.16 was 80 m²/g.

Ex. 17

(Preparation of Emulsion)

13 g of an EO-PO-EO block copolymer (PE10500 manufactured by BASF) wasadded to 174 g of pure water and dissolved. The solution was maintainedat 5° C., and 13 g of a PO-EO/PO block copolymer (25R-1 manufactured byADEKA Corporation) was added and dissolved to obtain a transparentaqueous solution. 300 g of pure water was heated to 60° C., to which theblock copolymer mixed solution was dropwise added with stirring, wherebya fine emulsion having a droplet size of about 300 nm was obtained.

First stage shell formation and subsequent steps were conducted in thesame manner as in Ex. 5 to prepare silica particles, except that Firststage shell formation was conducted at 60° C., and the liquid having thesilica material added was not maintained at pH 5, and a 1M sodiumhydroxide aqueous solution was added to pH 9.

The specific surface area of the hollow silica particles obtained in Ex.17 was 160 m²/g.

This application is a continuation of PCT Application No.PCT/JP2018/047619, filed on Dec. 25, 2018, which is based upon andclaims the benefit of priority from Japanese Patent Application No.2017-248972 filed on Dec. 26, 2017. The contents of those applicationsare incorporated herein by reference in their entireties.

What is claimed is:
 1. A method for producing hollow silica particles,which comprises: adjusting the pH of an oil-in-water emulsion containingan aqueous phase, an oil phase and a surfactant to at most 3.0 andadding a first silica material to the oil-in-water emulsion, adding asecond silica material to the emulsion having the first silica materialadded, at its pH of at least 8, in the presence of alkali metal ions, toobtain a hollow silica precursor dispersion, and obtaining a hollowsilica precursor from the hollow silica precursor dispersion andobtaining hollow silica particles from the hollow silica precursor. 2.The method for producing hollow silica particles according to claim 1,wherein the first silica material and the second silica material areeach independently at least one member selected from the groupconsisting of an alkali metal silicate, activated silicic acid and asilicon alkoxide.
 3. The method for producing hollow silica particlesaccording to claim 2, wherein the alkali metal silicate is sodiumsilicate.
 4. The method for producing hollow silica particles accordingto claim 1, wherein as the first silica material, an alkali metalsilicate aqueous solution is used.
 5. The method for producing hollowsilica particles according to claim 1, wherein as the second silicamaterial, at least one of an alkali metal silicate aqueous solution andan activated silicic acid aqueous solution is used.
 6. The method forproducing hollow silica particles according to claim 1, wherein thesecond silica material is added to the emulsion heated.
 7. The methodfor producing hollow silica particles according to claim 1, wherein anacid is added to the oil-in-water emulsion to adjust the pH to at least3, and then as the first silica material an alkali metal silicateaqueous solution is added.
 8. The method for producing hollow silicaparticles according to claim 1, wherein a base is added to the emulsionafter adding the first silica material, and then the second silicamaterial is added.
 9. The method for producing hollow silica particlesaccording to claim 1, wherein the hollow silica precursor is baked toobtain the hollow silica particles.
 10. The method for producing hollowsilica particles according to claim 9, wherein the baking temperature isfrom 300 to 800° C.
 11. The method for producing hollow silica particlesaccording to claim 1, wherein the surfactant is apolyoxyethylene/polyoxypropylene copolymer.
 12. The method for producinghollow silica particles according to claim 1, wherein the obtainedhollow silica particles have an average primary particle size of from 10nm to 10 μm and a BET specific surface area of at most 300 m²/g. 13.Hollow silica particles, having an average primary particle size of from10 nm to 10 μm and a BET specific surface area of at most 150 m²/g, andcontaining an alkali metal component in an amount of at least 500 massppm in the shell layer.